Precision resistance apparatus and methods of making



Sept. 17, 1963 JULIE 3,104,311

PRECISION RESISTANCE APPARATUS AND METHODS OF MAKING Original Filed March 29, 1957 2 Sheets-Sheet 1 .2, l 1 Tia 4 47 46-.44 40 T R 45 a T 30 42 1 V I L. JULIE Sept. 17, 1963 PRECISION RESISTANCE APPARATUS AND METHODS OF MAKING 2 Sheets-Sheet 2 Original Filed March 29, 1957 Tlcill. v

ATTORNEY United States Patent 3,104,311 PRECISIGN RESISTANCE APPARATUS AND METHUDS 0F MAKING Loebe Julie, New York, NY, assignor to Julie Research Laboratories, Inc, New York, N.Y.

Original application Mar. 29, 1957, Ser. No. 649,439, new Patent No. 3,047,826, dated July 31, 1962. Divided and this application Jan. 23, I961, Ser. No. 97,402

4 Claims. (Cl. 219-167) The present invention relates to precision resistance appar-atus and methods of making precision resistance elements. More particularly, the present invention relates to precision wire-wound resistance apparatus and to meth ods of making wire-wound resistance elements having precisely predetermined resistance values. The methods and apparatus described herein as illustrative embodiments of the present invention enable the fabrication of resistors in which the location of the terminations at the ends of the resistance wire is determined with an accuracy commensurate with the diameter of the wire and wherein the precision of the termination is maintained over indefinitely long periods of time both during use and during idleness. In typical resistors embodying the present invention the locations of the terminations are determined with a precision of better than one part per million of the total length of the resistance wire.

The instant application is a division of parent application Serial No. 649,439, filed March 29, 1957, entitled Precision Wire-Wound Resistance Apparatus and Resistors, and now US. Patent No. 3,047,826.

Among the many advantages of the methods and apparatus of the present invention are those resulting from the fact that they enable the fabrication of wire-wound resistance elements whose effective lengths are precisely determinable as may be desired prior to final assembly with a precision of better than one part per million and without requiring the use of auxiliary or trimming resistance elements to reach the desired values. In accordance with certain prior art practices in making encapsulated precision resistors it is necessary to utilize an expensive and time-consuming procedure. The first step of this prior practice is to try to make a main wire-wound resistor having a value as close to but below the desired value as possible; the resistance wire is subjected to an aging pro cedure to stabilize its resistance characteristics; then the actual value of this main resistor is measured after ass-embly and aging; next an auxiliary trimming resistor wire is added in series with the rnain resistor; and finally the total value of the composite resistor is measured, often following a second aging for the composite resistor.

In accordance with the methods and apparatus described herein as illustrative embodiments of the present invention a single resistance wire is utilized for each resistor. This wire is wound on the support, joined by butt-welding at one end to a terminal. Then it is aged by a suitable aging procedure as described below, and finally is cut to the desired length and its free end is butt-welded to the opposite terminal, providing a resistor whose actual resistance value after assembly is precisely determined in advance of final assembly and which is extremely stable in operation over an indefinitely long time.

Among the further advantages of the methods and apparatus of the present invention are their suitability for making individual highly precise wire-wound resistors having resistance values of any desired amount within the entire range from ten ohms up to ten million ohms and which maintain their efiective points of termination precisely positioned so that the resistors are stable in their resistance values over indefinitely long periods of time.

In addition, these resistance elements exhibit relatively high mechanical strength and ability to withstand vibra- "ice tion. The methods and apparatus of the present invention enable the elimination of the complex arrangements and systems required by the prior art to secure precise results.

Among the advantages of the butt-welding methods and apparatus of the present invention is the fact that they enable the location of the eifective point of connection at each end of the wire with respect to the length of the resistance wire to be determined with an absolute precision better than the diameter of the wire itself. In prior resistors the ends of the resistance wires are connected by arrangements which leave the location of the effective points of termination indefinite by amounts of thirty diameters of wire or more, for example, by beads of solder or the like. My tests have indicated that the effective point of connection between a high resistance wire and a relatively low resistance solder bead tends to travel along the length of the wire with the passage of time and with passage of current, thus changing the effective length of the resistance wire which is in circuit between its terminals. The eliective lengths of the resistance wires in circuit between the terminals advantageously remains constant in the illustrative embodiments of the invention described herein.

These resistors described herein lend themselves readily to all of the conventional packaging techniques such as oil immersion, potting, wax coating, and encapsulation, and also to the improved encapsulation procedures described herein as illustrative embodiments of the present invention. This encapsulation procedure provides a completely sealed pocket enclosing the resistance wire.

' The main body of the resistance winding remains entirely free of the encapsulating material and yet is entirely protected thereby. In certain instances this pocket contains air or suitably treated gases and in other instances this pocket is filled with a protective oil which bathes the resistance wire and further protects it. The fluid, gas or liquid, within this pocket thus advantageously isolates the resistance wire from mechanical stresses which may be present in the encapsulating material and by convection serves to cool the resistance wire.

In this specification and in the accompanying drawings, are described and shown methods and apparatus embodying my invention and Various modifications thereof are indicated, but it is to be understood that these are given for purposes of illustration in order that others skilled in the art may fully understand the invention and the manner of applying the methods and apparatus in practical use so that they may modify and adapt them in various forms, each as may be best suited to the conditions of a particular electrical application.

The various objects, aspects, and advantages of the present invention will be more fully understood from a consideration of the following specification in conjunction with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a precision resistor embodying the present invention wherein the ends of the resistance Wire are butt-welded to the terminals;

FIGURE 2 illustrates a method of making the buttwelded connection at one end of the resistance wire;

FIGURE 3 is a perspective view of another embodiment of the present invention utilizing a different form of butt-welded terminal connection for the resistance wire;

FIGURE 4 is an enlarged elevational view shown partially in section, illustrating the terminal connection;

FIGURE 5 is a View similar to FIGURE 2 illustrating a different terminal connection and method of making;

FIGURE 6 is an enlarged elevational view shown partially in section of a resistor incorporating a different of butt-welded end termination;

FIGURE 7 is an end elevational view of the resistor of FIGURE 6;

of encapsulated precision resistor embodying the present invention and show steps in the methods of fabrication.

7 FIGURE 14 is a cross sectional view of FIGURE 13 taken along the line 14-14; and

FIGURE 15 is. adaigrammatic illustration of the shortcomings I have found to be present in prior art terminations of the soldered type. V g

The resistor '26 shown in FIGURE 1 includes a ceramic spool or bobbin 21 having five flanges 22 integrally formed on a generally cylindrical hollow winding core.

support 24. The resistance wire 26 is wound on the cylindrical support 24 with approximately equal lengths lying in each of the four annular winding channels 26, 27, 28, and 29 between the respective flanges.

In order to reduce the effective inductance of the resistance winding, the directions of winding of the portions of the wire lying in adjacentwinding channels is reversed.

a In passing from one winding channel to the next, the

wire runs through a slot 30 in the intervening flange and reverses its direction as shown in the drawing, in entering the next channel. The slots 30; extend the full depth of the flanges and are axially aligned.

' A pair of terminals 31 and 32 are fixed at opposite ends of the bobbin. Each of these terminals includes an extending outer contact lug, 34 for making external electrical connections and for mounting the resistor. The inner ends'of these terminals have fastening rings 36 d thefree end of the wire to the desired length and weld it to the opposite lug piece it For example, in making a butt-weld at the second end of the wire when the.

winding operation has been completed as shown in FIG- URE 2, the wire must be trimmed was to give the precisely desired length after it has been welded to the lug 49. In order to obtain this precisely desired length after welding, a small allowance of' an added increment of length must be made for wire which becomes fused and thus consumed in the welding operation. This allowance is less than .005 of an inch.

After trimming, the enamel or other insulation material is scraped from or otherwise removed from the wire end as shown at 44, for a length of about one-quarter of an inch. The bared wire is grasped between a pair of electrically conductive. tweezersv 46 connected by a which snugly fit around the extending end portions of V the core 24. For purposes of holding these terminals 31 and 32 firmly in place, a C-shaped retaining clip 38 is snapped into a groove in the extending end of the core against the outside of the ring portion 36. In this illustrative embodiment of the invention, the terminals are shown as being tin coated electrical copper material. The resistance wire 26 is insulated resistance wire such as is commercially available and specific examples of suitable wire are discussed in detail further below.

In order to make a connection at the ends of the wire 26, as illustrated in FIGURE 2, a butt-weld is formed between each end of the resistance wire and a terminal lugs of suitable material. For example, this terminal lug 40 is formed of Phosphor-bronze and is suitably'secured, for example, by soldering or brazing, at 42 to a tab 43 which projects on the opposite rim of ring 38 from the external terminalend34. The purpose of this lug 40 is to provide suitable material to which to butt weld the end of the resistance wire. Phosphor-bronze or nickel material serve extremely well for this use. It is also possible to use a terminal material which is the same as that of the resistance wire. However, for most applications Phosphor-bronze material has been found to be most satisfactory from all considerations including its ease of fabrication and handling.

In certain instances the entire terminal 31 or 32 can be stamped out of Phosphor-bronze or other suitable material. However, the arrangement as illustrated is very satisfactory. The lug piece 40 serves as a transition between the high resistance end of the resistance wire and the highly conductive material of the terminals 31 and 3 2. 7 I

lead 47 in circuit in serieswi-th a current limiting resistor 48 and a charged capacitor 50'. A lead 51 completes the circuit from the opposite side of this capacitor to the terminal 32. The tweezers are used to touch the end of the wire 26 substantially perpendicularly against the clear surface of the lug 40. An arc is created at the end of the wire as the capacitor 50 is discharged. The magnitude of the current flow through the arc is limited by the resistor 48 to obtain the desired welding action.

As a result of this process, a precisely controlled weld is obtained and the desired value of the resistance wire unction with these materials:

. Wire Lug Voltage Capaci- Resist- Wire Material Diam. Material V t-ance ance R,

' 0, pt. ohms Evanohm .001 Ph-B 300 1 25 DO .002 Ph-B 300 2 25 004 Ph-B 300 8 25' 001 Ph-B 300 1 25 002 Ph-B 300 2 25 004 Ph-B 300 8 25 001 Ph-B 300 1 50 002 Ph-B 300 2 50 004 Ph-B 300 8 50 001 Ph-B 300 1 50 002 Ph B 300 2 50 004 Ph-B 300 8 50 Evanohm enamel-coated resistance wire is obtainable commercially from Wilbur B. Driver Co. of Newark, New Jersey and Karma enamel-coated resistance wire from Driver-Harris Co. of Harrison, New Jersey. Suitable resistance wires, such as these, have a composition iron. Another very suitable resistance wire is sold by Driver-Harris Co. under the name Nichrome and has a a composition of approximately 60% nickel, 15%, chro- It will be noted that the lug piece 40 is secured to the mium and 25% iron. These all require approximately a 25 ohm current limiting resistor.

Manganin enamel-coated resistance wire is obtainable commercially from Wilbur B. Driver Co. and Advance from Driver-Harris and they require approximately a 5-0 ohm current limiting resistor.

Byfollowing this procedure the operator is enabled to determine in advance of butt-welding the eifective point of connection to the terminal with a precision tolerance commensurate with the diameter of the wire'itself,

usually resulting in an over-all precision of better than one part per million in resistance value. Because the location of the effective points of terminal connections are thirty times more precisely determined, this invention enables the making of resistors containing only onethirtieth the length of wire and yet having a precision equal to or better than prior resistors. shorter wires used, these resistors have far less inductance and capacitance than pnior resistors of the same precision.

For resistors having a resistance value below 10,000 ohms, it is usually preferable to utilize a resistance wire having a diameter of at least .004 of an inch. In the range between 10,000 ohms and 100,000 ohms, a diameter of .002 of an inch is preferable; and about 100,000 ohms, .00 1 of an inch is preferable.

In the remaining figures of the drawings corresponding reference numerals are used for parts performing corresponding functions. Parts performing similar functions have the same reference numeral followed by an appropriate letter. As shown in FIGURES 3 and 4 the bobbin 21a is similar to the ceramic bobbin 21, but is formed of an epoxy resin material. The resistance wire 26 is passed through aligned slots 30 in the flanges 22.

In order to terminate the resistance wire, the bared end portions 44 are butt-welded to the clear inner surfaces of short lug rods 40a of Phosphor-bronze, nickel, or other suitable material whose outer ends are soldered at 4-2 to the respective ring portions 36 of the external terminals 3 1 and 32. These terminal lugs 40a extend inwardly through holes 52 in [the respective end flanges 22a and project inwandly a short distance from the inner surface of the flange for accessibility in making the butt-weld.

'Ilhe resistor of FIGURE is generally similar to that shown in FIGURES 1 and 2 except that the lug piece 40 is just slightly narrower than the slot 30 in the end flange 22. The butt weld connection between the bared end 44 and the lug piece 40' is made near the free end of this lug piece. After the weld is completed, the lug piece 40 is bent inward through the slot 30 so that it becomes depressed below the perimeter of the bobbin flanges. Dotted lines indicate the position of welded wire end 44 and lug piece 40 prior to the bending operation.

In the high precision resistor of FIGURES '6 and 7 the bobbin 21b is of epoxy resin and is identical with that shown in FIGURES 3 and 4 except that the end flanges 22 are slotted at 30 instead of having terminal lug holes as at 52 in FIGURE 3. To form the termination for the resistance wire, a short lug rod 40b is used having a diameter snugly fitting into the slot 30. This lug rod 401) is secured to the inner face of the terminal tab 43 and advantageously acts to plug up the slot 30,. which is helpful in the encapsulation discussed below. The butt-welded connection is made to the inner side of the cylindrical surface of this radially extending lug rod 4%.

The assembled resistors in FIGURES 3-7, as shown lend themselves to the encapsulation steps described hereinafter, because none of the terminal portions project beyond the periphery of the flanges except for the two outer terminal end connections 34. Moreover, the slots 30, or holes 30w, as the case may be, in the end flanges are purposefully obstructed by the arrangement of the terminals themselves, for reasons explained below.

The encapsulating material which is described herein by way of example is epoxy resin. So, it is more advantageous to use a spool or bobbin 21a or 21b of epoxy material, whereby the temperature expansion coefficients of en capsulating material and bobbin match. However, by virtue of the fact that the resistance wire is bathed in a fluid within a pocket in the capsule, it is isolated from any undesirable effects arising from stresses or strains in the encapsulating material itself. Thus, a ceramic bobbin 21 also can be used.

As a first step in the encapsulation procedure, the outer ends 34 of the terminals 3 1 and 32 are held upwardly and an impervious plastic film 56 is applied as a cylindrical sleeve tightly embracing the flanges 2 2 and forming a By virtue of the completely enclosed annular pocket 58 Within the sleeve 56. This pocket is divided into four parts by the intervening flanges 22, but these all communicate with one another through the slots 30 in the flanges.

A convenient way to form this impervious sleeve 56 is to wind around two or three layers of pressure-sensitive adhesive-coated Mylar tape, or cellophane tape, such as Tuck tape or Scotch tape, having a width matching the distance between the end flanges.

Then the resistor is encapsulated in epoxy resin 59, as shown in FIGURE 9-, forming a protective capsule for the resistor unit and providing added mechanical support for the terminals 31 and 32. Entry of any of the encapsulating material into the pocket 58 is prevented by the terminals which obstruct the openings in the end flanges.

As a final step, the ends of the capsule are cut off along the planes 61 and 62 perpendicular to the axis of the bobbin and flush with the ends of the hollow core 24. This minimizes the axial length of the units and facilitates their end-to-end mounting. Where desired a non-magnetic mounting rod is passed through the hollow core 24.

As a result of this encapsulation procedure air is the fluid medium which bathes the resistor wire within the enclosed pocket 58. By convection this fluid aids in carrying away heat from the wire when in use. To obtain a dry air bath, a suitable desicant, such as silica gel, is introduced into the pocket before the tape 56 is wound in place.

FIGURES ll14 illustrate a method of fabrication wherein transformer oil is utilized substantially filling the annular pocket 58. Aiiter the impervious sleeve 56 is applied, epoxy resin is cast around the resistor up to a level 65 approximately two-thirds way between the top of the winding 26 and the periphery of the flanges 22, so as to leave only a narrow portion of the sleeve 56 exposed at the top. This casting of the encapsulating material 50 includes the steps of pouring it into a mould around the sleeve 56 and curing it. Then, after curing the material 59, a hypodermic 6-6 is utilized to inject oil into the pocket 58 through the remaining narrow exposed portion of .the sleeve 56. This flows through the slots 30, which are aligned at the lowermost point because the terminals 31 and 32 are held up vertically, and thus the oil substantially entirely fills the pocket 58. The displaced air escapes through the needle hole in the sleeve 56, which is purposely somewhat enlarged, and through small pin holes over the other winding channels. Thereafter, as shown in FIGURES l3 and 14 further resin material 59' is cast above the level 65 to complete the encapsulation of the unit. The holes in the sleeve 56 used to inject the oil are suitably plugged, for example, they are covered with a patch of plastic tape before the material 59 is cast in place. Fin-ally, the excess encapsulating material is cut off from the ends flush with the ends of the core 24. Suitable epoxy encapsulating material is obtained from Houghton Laboratories, Inc. of Olean, New York.

A suitable aging procedure for stabilizing the resistance characteristics of the wire includes the following steps:

A. (l)Maintain 2 hours at 0 C.

(2)Maintain 2 hours at -50 C. (3)Maintain 2 hours at C.

Repeat these three steps in sequence five times each.

B. (1)Maintain rated current through the wire for 24 hours steadily. (2)No current through the wire for 24 hours.

Repeat these two steps in sequence five times each.

C. Repeat A steps in sequence five times each. D. Allow to stand at room temperature for 2 months.

Highly suitable ceramic bobbins 21 are obtainable from Thor Ceramics, Inc. of Bloomfield, New Jersey, and epoxy '2" resinbobbins from Norrich Plastics Corp, of New York city, New York.

To emphasize further the advantages of the butt-welded termination for the wire, attention is directed to FIG- URE 15 showing a resistance wire 70 wrapped around a bifurcated terminal lug '72 and encased in a solder bead 74. it will be appreciated that the specific resistivity of the wire 70 is many times larger than that of the, bead. Thus, the effective point of connection of the wire 70 to the terminal 72 is the point on the wire furthest. from the terminal at which a good electrical conneotion exists, between the wire 76 and the head 7 Initially this effective point of contact is at 76 at the surface of the solder bead. However, by some deteriorating action or other such as oxidation, 1 find that a barrier to conduction builds up around the high resistance.

wire at its point of entry .76. And so, the effective point of contact begins to creep inwardly along the length of the wirewithin the solder, as indicated by the arrow. At some subsequent period of time this effective point of contact'often will have moved to'a point 78 which is a substantial distance from the point of entry at 76'. As a result, the length of wire between 76 and 7% is added to the resistor, and this can often amount to more than one-tenth of an inch. Moreover, this movement of the effective contact point is erratic, and varies with use and time, sometimes jumping ahead or retracing backwardly, causing a varying resistance value. None of these undesirable efiects are present in the butt-welded terminations described herein. a

From the foregoing it will be understood that the embodiments of the precision resistance apparatus and methods of making of the present invention described above are well suited to provide the advantages set forth,

be interpreted as illustrative and not in a limiting sense and that in certain instances, some of the features of the invention may be used without a corresponding use of other features, all without departing from the scope of the invention.

What is claimed is:

resistor comprising the steps of butt-welding one end of the resistance wire to a terminal, winding the resistance wire onto a suitable support, trimming the free end of the wire to the desired length plus an added increment of length of less than .005 of an inch, and butt-welding the trimmed end to a terminal.

2. The method of making a high-precision wire-wound resistor having a pre-determined resistance value comprising the steps of winding the resistance wire upon an insulating support, 'butt welding one end of the resistance Wire to a termination at one end of'the support, trimming the other end of the resistance wire to the desired length for providing said resistance value plus an added increment of length of less than .005 of an inch, and butt welding this trimmed end to a termination at the other length compensates for wire loss u-pon flash-welding said free end to a second terminal, providing a second terminal having a relatively flat [area portion substantially larger than the cross-sectional butt-end area of said free end, and butt-connecting said wire free end to said larger area portion of said second terminal by flash-welding same. V

4. A method as defined in claim 3 wherein, the buttconnect-ing step includes the steps of connecting a charge capacitor in a circuit between the terminal free end of said resistance wire and said second terminal, and subsequently movingthe sa'id terminal free end of wire towards the large area surface of said second terminal to effect a perpendicular butt-flash weld thereat.

References Cited in the file of this patent UNITED STATES PATENTS Ewald Mar. 1, 1960 

1. THE METHOD OF MAKING A HIGH PRECISION WIRE-WOUND RESISTOR COMPRISING THE STEPS OF BUTT-WELDING ONE END OF THE RESISTANCE WIRE TO A TERMINAL, WINDING THE RESISTANCE WIRE ONTO A SUITABLE SUPPORT, TRIMMING THE FREE END OF THE WIRE TO THE DESIRED LENGTH PLUS AN ADDED INCREMENT OF LENGTH OF LESS THAN .005 OF AN INCH, AND BUTT-WELDING THE TRIMMED END TO A TERMINAL. 